"Fully sick", "hectic", "wicked" or "that's nice, dear".
How you say "that's really good" is up to you, but you'll say it when you start plugging your guitar into effects and pedals. You won't believe your guitar can sound like this: crunching metal, solos that sound like you're in an arena, warm echoes and so much more. It's how the big guys and girls get their big sounds. We've got stacks in our Mega Music stores and the only way to hear what they can do is to come in, plug in and have a play-time. Sweeeet! Delightful! Awesome! And so forth.